The invention relates to a dispersion compensated multimode waveguide link which includes a compensating multimode optical waveguide fiber which is optically coupled to the link to increase bandwidth at one or more preselected wavelengths.
Multimode optical waveguide fiber has long been preferred for use in shorter link length systems, such as local area networks, in which the link length is typically less than 5 km and the data transmission rate is of the order of hundreds of Mbits/sec. The large core diameter of multimode waveguides, typically 50 xcexcm, 62.5 xcexcm, 100 xcexcm or larger, allows for low loss splicing and connecting loses. In addition, multimode waveguides provide for operation at two wavelength windows, centered around 850 nm and 1300 nm, and have sufficient bandwidth at both wavelengths to meet local area network data rate requirements.
Because the waveguide attenuation at the 1300 nm window of operation is lower, the multimode waveguide manufacturing process may be adjusted to provide higher bandwidth at the higher wavelength window. This adjustment provides a higher wavelength window capable of carrying higher data rates over longer distances, in comparison to the lower wavelength window. In this way full use is made of the lower attenuation at 1300 nm. Thus, for example, multimode fibers having a bandwidth of 160 MHz-km at 850 nm and 500 MHz-km at 1300 nm (160/500 fiber) has been specified for many local network or other short length applications.
Applications in which bandwidth is optimized at lower operating windows is required in certain systems. In these cases, the wavelength of peak bandwidth is moved to a lower wavelength such as 780 nm or 850 nm.
However, as laser sources at the lower window have become more powerful, narrower in line width, and relatively free of chirp, a need has arisen for higher bandwidth in the wavelength region centered on 850 nm. In addition, for certain local area net applications the demand for increased bit rate continues. Thus a practical need has arisen for higher bandwidth in the 850 nm window while maintaining sufficient bandwidth in the 1300 nm window.
Because many networks have been installed using the two window bandwidth values of 160 MHz-km and 500 MHz-km at the respective 850 nm and 1300 nm windows, a search has been undertaken to find an economically feasible way to adjust or compensate the two window bandwidths in installed multimode waveguide fiber links.
Refractive Index profile is a statement of the value of the refractive index of a material along a line having a first and a last point. In the case of an optical waveguide fiber, a value of refractive index profile is defined at each point along a waveguide radius.
A general expression for an index profile is,
n2(r)=n12[1xe2x88x922xcex94f(r/a)],
in which.
n(r) is the refractive index at a point r on the waveguide radius, n1 is the refractive index on the waveguide centerline, xcex94=(n12xe2x88x92n22)/2n12, in which n2 is a reference refractive index usually taken as the minimum value of the clad layer index, and f(r/a) is a function of r divided by the core radius a.
An xcex1-profile is a refractive index profile in which f(r/a)=(r/a)xcex1.
Bandwidth is a standard measure of the dispersive property of a waveguide over a range of frequencies. In particular, the bandwidth of a waveguide is the range of frequencies over which the power penalty due to dispersion is less than 3 dB where the launched power is used as the comparison base. Bandwidth may be expressed in normalized frequency units, MHz-km, which is the bandwidth of a 1 km length of waveguide. When the bandwidth units are expressed simply as MHz, the bandwidth value is representative of the total length of waveguide measured. For example, a waveguide which is 2 km in length, having a normalized bandwidth of 500 MHz -km, has an end to end bandwidth of (500 MHz-km)/2 km=250 MHz.
There is a need for a technically sound, low cost way to compensate the bandwidth at one of the two operating wavelength windows. Further, this need exists in certain applications to compensate one wavelength window without unduly sacrificing bandwidth at the other wavelength window. It is contemplated that a refractive index profile n(r) may be found which has a local maximum near a selected wavelength window to be compensated and a local maximum at another selected wavelength window of operation. The present invention meets the need for such a multimode link bandwidth compensator.
A first aspect of the invention is a dispersion compensated multimode link comprising a first multimode fiber length, which has an index profile which provides for respective preselected bandwidths at a first and second wavelength window. The multimode waveguide has a core region and a surrounding clad layer. The core region has a circular cross section of radius a, the radius measured from the waveguide centerline. In a shorthand notation, the multimode waveguide is said to have bandwidth BW1 at wavelength xcex1 and a bandwidth BW2 at wavelength xcex2. Although the profile n1(r) may take many forms, the profile in general produces a bandwidth vs. wavelength curve which has a local maximum at a wavelength xcexp1. A respective target bandwidth at each of two operating windows is realized by a combination of the location of xcexp1 relative to xcex1 and xcex2, and the maximum bandwidth which is located at wavelength xcexp1. In order to reach the respective target bandwidths at each of the two windows, the profile is designed such that the maximum or peak bandwidth occurs at a wavelength xcexp1 which lies between the center wavelengths of the two operating windows, xcex1 and xcex2. The bandwidth vs. wavelength response of the waveguide may be calculated from the geometry and index profile of the waveguide. The mathematical relationships are quite complex even when mode coupling and mode mixing are not taken into account. Even using numerical methods and a computer, usually some simplifying assumptions must be made. Thus the term xe2x80x9cmathematically derivablexe2x80x9d is used in this document to mean a particular set of waveguide fiber properties, specifically the refractive index profile and the core and clad geometry, can be used;
to estimate relative mode delay,
to predict xcexp, or
to estimate bandwidth at xcexp.
The agreement between calculated and experimental compensation waveguide parameters, given below, demonstrates the validity of the assumptions used in this application.
The compensated link is completed by optically joining a second multimode waveguide fiber to the first fiber. The second multimode waveguide has an index profile, n2(r) which compensates the relative modal delays which occur in the first waveguide. One method of compensation makes use of a compensating waveguide which has maximum bandwidth at a wavelength, xcexp2. An example of such a profile is an a profile. By placing xcexp2 outside the wavelength interval defined by xcex1 and xcex2 one of the bandwidths, that of the higher wavelength window or that of the lower wavelength window can be compensated by the second fiber. In the case in which the profile errors in the first waveguide are xcex1 errors, the compensating waveguide cannot in general correct the group delay of the modes at both xcex1 and xcex2 so that the increase in one bandwidth is made at the expense of the other. This is because the change in a produces a change in xcexp and so shifts the bandwidth vs. wavelength curve toward a higher or the lower wavelength.
When the profile errors which reduce bandwidth are more random or non-xcex1 in nature, it is possible to compensate both the high wavelength window bandwidth and the low wavelength window bandwidth with a single compensating waveguide. Thus, in general it is proper to stipulate that at least one bandwidth may be compensated. An alternative statement is that the compensating waveguide may function to equalize mode group delay at more than one wavelength.
Note that the compensated bandwidths, BWcomp1 and BWcomp2 are expressed in MHz. In this way, the end to end bandwidth of the waveguide fiber is compared before and after compensation. The compensation must be sufficient to improve the end to end bandwidth in MHz even though the link has been made longer by the compensating waveguide.
In a preferred embodiment of the compensated multimode link, either the first or second multimode waveguide has an xcex1-profile, defined above, in which xcex1 lies in the range of about 0 to 8. This choice of index profile allows one to equalize mode group delay at a selected wavelength and is flexible enough to provide acceptable bandwidth for operation at both the 1300 nm and 850 nm window. The calculation of compensation waveguide parameters is also made easier by the choice of xcex1 profiles.
In a most preferred embodiment, both the first and second multimode wavelengths comprising the link have respective xcex1 profiles. For this refractive index profile choice, working relationships among the parameters of the compensated link, the first waveguide, and the second waveguide take a particularly simple form.
A first waveguide fiber having an a xcex1=xcex11 in the range 0.8xe2x89xa6xcex11 less than 2.1, may be compensated by a second waveguide fiber having an a xcex1=xcex12 in the range xcex11xe2x89xa6xcex12xe2x89xa68. This choice serves to increase the bandwidth at a lower wavelength operating point such as 850 nm. Using the same xcex11 for the first fiber, the bandwidth at a higher wavelength operating point, such as 1300 nm, may be increased by using a second, i.e., compensation, waveguide having an xcex12 in the range 0.8xe2x89xa6xcex12xe2x89xa6xcex11.
Defining xcex11 as characteristic of the first waveguide, xcex12 as characteristic of the second or compensating waveguide, and xcex1comp as characteristic of the compensated link, then, for increasing the lower wavelength bandwidth, xcex1lcomp=(xcex11+cxcex12)/(1+c), in which, c=L2/L1, where L1 is the length of the first waveguide, L2 is the length of the second waveguide, and c is a number in the range 0 to 1. Likewise, if the higher wavelength bandwidth is compensated, xcex1hcomp=(xcex11+cxcex12)/(1+c).
In a second aspect of the invention, the refractive index profile of the first length, L1, takes the form, n1(r)=ncl12[1xe2x88x922xcex941f1(r/a1)], in which, ncl1 is the refractive index on centerline and a1 is the core radius. The relative index xcex941 is referenced to the minimum value of the clad layer refractive index nc1. The second multimode length has an index profile of the same form, n22(r)=ncl22[1xe2x88x922xcex942f(r/a2)]. The combination of the first and second fiber may be chosen to compensate bandwidth at either a higher or lower operating wavelength.
In a preferred embodiment, either f1(r/a1) or f2(r/a2) has the form (r/a)xcex1, i.e. an xcex1 profile.
In a most preferred embodiment, both the first and second waveguides have respective xcex1 profiles. The limits and relationships of the respective xcex1""s, xcex11 for the first fiber, xcex12 for the second fiber, and xcex1comp for the combination of the two fibers, are as stated above.
The most advantageous operating windows for multimode waveguide fiber are centered at 850 nm and 1300 nm. At these wavelengths, and in an interval +/xe2x88x9230 nm about these wavelengths, the attenuation vs. wavelength characteristic shows a local minimum.
In a particular embodiment of the invention, a link having xcexp1 in the range of about 1150 nm to 1250 nm, is compensated at the 850 nm operating point by optically joining a compensating waveguide having xcexp2 in the range of about 450 nm to 650 nm. Further characteristics of this embodiment are given in an example below.
In keeping with the purpose of the compensated link, i.e., to increase data rate, the preferred embodiments of the invention are those in which the length of the compensation or second waveguide is minimum. Thus a preferred embodiment of the invention is one in which the xcex1 of the compensation fiber is large compared to that of the first fiber. The larger xcex1 provides equal compensation using shorter compensation lengths as compared to a compensation waveguide of lower xcex1. The fractional length c, c=L2/L1, which is the range 0 to 1, has a preferred range of 0.01 to 0.50. In cases of xcex1 in the range of about 2.5 to 3, the preferred range of c is reduced to about 0.01 to 0.25.